This invention is concerned with the manufacture of high quality lubricating oils, and in particular with lubricating oils derived from petroleum distillate fractions. It is especially directed to the preparation of low pour point lubricating oils that have a high V.I. (V.I. will be used herein to denote "viscosity index") from crude oils of high wax content. This invention is specifically directed to catalytically dewaxing a waxy distillate lubricating oil utilizing as catalyst the zeolite ZSM-57, optionally associated with a hydrogenation component, to obtain a lubricating oil of low pour point and high V.I.
Refining suitable petroleum crude oils to obtain a variety of lubricating oils which function effectively in diverse environments has become a highly developed and complex art. Although the broad principles involved in refining are qualitatively understood, the art is encumbered by quantitative uncertainties which require considerable resort to empiricism in practical refining. Underlying these quantitative uncertainties is the complexity of the molecular constitution of lubricating oils. Because lubricating oils for the most part are based on petroleum fractions boiling above about 450.degree. F., the molecular weight of the hydrocarbon constituents is high and these constituents display almost all conceivable structure types. This complexity and its consequences are referred to in well known treatises such as "Petroleum Refinery Engineering," by W. L. Nelson, McGraw Hill Book Company, Inc., New York, N.Y., 1958 (Fourth Edition).
In general, the basic premise in lubricant refining is that a suitable crude oil, as shown by experience or by assay, contains a quantity of lubricant stock having a predetermined set of properties such as appropriate viscosity, oxidation stability and maintenance of fluidity at low temperatures. The process of refining to isolate that lubricant stock consists of a set of subtractive unit operations which removes the unwanted components. The most important of these unit operations include distillation, solvent refining and dewaxing which basically are physical separation processes in the sense that if all the separated fractions were recombined one would reconstitute the crude oil.
A refined lubricant stock may be used as siuch as a lubricant, or it may be blended with another refined lubricant stock having different properties. Or, the refined lubricant stock, prior to use as a lubricant, may be compounded with one or more additives which function, for example, as antioxidants, extreme pressure additives and V.I. improvers.
For the preparation of a high grade distillate lubricating oil stock, the current practice is to vacuum distill an atmospheric tower residuum from an appropriate crude oil as the first step. This step provides one or more raw stocks within the boiling range of about 450.degree. to 1050.degree. F. After preparation of a raw stock of suitable boiling range, it is extracted with a solvent, e.g., furfural, phenol, sulfolane or chlorex, which is selective for aromatic hydrocarbons, and which removes undesirable components. The raffinate from solvent refining is then dewaxed, for example, by admixing with a solvent such as a blend of methyl ethyl ketone and toluene. The mixture is chilled to induce crystallization of the paraffin waxes which are then separated from the raffinate. Sufficient quantities of wax are removed to provide the desired pour point for the raffinate.
Other processes such as hydrofinishing or clay percolation may be used if needed to reduce the nitrogen and sulfur content or improve the color of the lubricating oil stock.
Viscosity index (V.I.) is a quality parameter of considerable importance for distillate lubricating oils to be used in automotive engines and aircraft engines which are subject to wide variations in temperature. This index indicates the range of change of viscosity with temperature. A high viscosity index of 100 indicates an oil which resists becoming viscous at low temperature or becoming thin at high temperatures. Measurement of the Saybolt Universal Viscosity of an oil at 100.degree. and 210.degree. F., and referral to correlations, provides a measure of the V.I. of the oil. For purposes of the present invention, whenever V.I. is referred to it is meant the V.I. as noted in the Viscosity Index tabulations of the ASTM (D567), published by ASTM, 1916 Race St., Philadelphia 3, PA., or equivalent.
To prepare high V.I. automotive and aircraft oils, the refiner usually selects a crude oil relatively rich in paraffinic hydrocarbons since experience has shown that crudes poor in paraffins, such as those commonly termed "naphthene-base" crudes, yield little or no refined stock having a V.I. above about 40. Suitable stocks for high V.I. oils, however, also contain substantial quantities of waxes which result in solvent-refined lubricating oil stocks of high pour point. Thus, in general, the refining of crude oil to provide acceptably high V.I. distillate stocks ordinarily includes dewaxing to reduce the pour point.
In recent years, catalytic techniques have become available for dewaxing of petroleum stocks. A process of that nature developed by British Petroleum is described in The Oil and Gas Journal, dated Jan. 6, 1975, at pages 69-73. See also, U.S. Pat. No. 3,668,113.
U.S. Pat. No. Re. 28,398 (of original U.S. Pat. No. 3,700,585) describes a process for catalytic dewaxing employing a catalyst comprising zeolite ZSM-5. Such a process combined with catalytic hydrofinishing is described in U.S. Pat. No. 3,894,938. U.S. Pat. No. 3,755,138 describes a process for mild solvent dewaxing to remove high quality wax from a lube stock which is then catalytically dewaxed to specification pour point. The entire contents of these patents are incorporated by reference herein.
It is interesting to note that catalytic dewaxing, unlike prior-art dewaxing processes, although subtractive, is not a physical process but rather depends on transforming the straight chain and other waxy paraffins to non-wax materials. The process, however, is more economical and thus of industrial interest even though at least some loss of saleable wax is inherent. Commercial interest in catalytic dewaxing is evidence of the need for more efficient refinery processes to produce low pour point lubricants.